Electronic devices commonly consist of a variety of electronic components formed in or mounted on some sort of substrate. Among other things, the substrate provides a platform to which the components can be mounted and also provides electrical connections between and among the components.
FIG. 1 illustrates an embodiments of a substrate 100. The substrate 100 includes a plurality of conductive layers 102, as well as a plurality of electrically insulating inter-layer dielectric (ILD) layers 104 that separate the conductive layers 102 and electrically insulate them from each other. The substrate 100 is typically built by depositing alternating conductive layers 102 and ILD layers 104 until a substrate is created with the number of layers needed for the required electrical interconnects. The ILD layers 104 are made up of some form of dielectric material. In some embodiments, the material will be some sort of polymer or polymer-based material; the particular material chosen for the dielectric will depend on such factors as the required dielectric constant k, and the physical properties required for manufacturability. Although not shown in FIG. 1, both the conductive layers 102 and ILD layers 104 can have holes of vias therein. When filled with a conducting material, for example, the vias allow electrical connection and communication between different conductive layers within the substrate. Shrinking electronic packages and rising power requirements, along with the advent of ILDs having a low dielectric constant (i.e., low-k ILDs) are forcing dielectric build-up layers 104 to ever-increasing levels of performance.
Over the lifetime of the substrate 100, various factors may contribute to a gradual degradation of the substrate or its individual layers. One important factor that degrades the performance of the ILD layers 104 is the migration into and through the ILD's dielectric material of ions and molecules—most notably, but not exclusively, water, oxygen, halogen ions and metal ions. These molecules may originate from environmental sources outside the substrate 100, as shown by arrows 106. Molecules and ions, in particular metal ions, may also originate from within the conductive layers 102, or along the interface between a conductive layer 102 and an ILD layer 104 as shown by arrows 108. Finally, molecules and ions may originate from within the ILD layer 104 itself: halogen ions and oxygen, for instance, may be released from the ILD material by a process known as “outgassing.” Although in FIG. 1 the arrows 108 and 110 only illustrate migration of molecules and ions in one direction, the molecules and ions may migrate into and through the ILD in the opposite direction as well.
FIG. 2 illustrates an embodiment 200 of the attachment of an electronic device to a substrate. A die 202 is attached to a substrate 204 using a plurality of solder balls 206. In addition to anchoring the die 202 to the substrate, the solder balls 206 provide an electrical connection to an underlying conductive layer 212 within the substrate. In assemblies such as the one shown, it is common to put a layer of solder resist 208 on the surface of the substrate before attaching the die 202 using the solder balls 206. As its name implies, the purpose of the solder resist is to resist the solder; among other things, the solder resist prevents the solder from flowing onto and into portions of the substrate where it is not wanted, and prevents electrical connection between the solder balls and other areas of the substrate. The solder resist layer 208, like the ILD layers 210 within the substrate, can be made of a dielectric material and it therefore suffers from degradation over time due to the same molecular and ionic migration problem that the ILDs suffer from—namely, the migration of ions and molecules such as water, oxygen, halogen ions and metal ions. As with the ILDs, the ions or molecules entering into or migrating through the ILD can originate externally, such as from the environment or from adjacent layers of the substrate, or can be internal, by processes such as outgassing.
Current solutions used to reduce molecular and ionic migration through ILD and solder resist layers center around chemical modification of the ILD or solder resist polymer, for example by using more hydrolytically and oxidatively stable monomers, etc. Often silica is added to toughen the polymer and lower the coefficient of thermal expansion (CTE) of a solder resist layer. None of these solutions, however, substantially slow or stop the migration of ions and molecules through the material and, consequently, do little to counter the degradation that occurs over time because of these ionic and molecular migrations.